The surface commensal microbial community has been shown to influence the function and development of the immune system, thus potentially participating in the pathophysiology of several important diseases of the skin, gut, kidney and lung. With increased appreciation that normal microbial communities exist in equilibrium with, and in many cases benefit the host, recent work has sought to better understand the mechanisms by which we permit survival of our skin commensal microbiome and how the microbiome controls mammalian immune responses. We have discovered that it was wrong to assume that bacteria are normally totally separated from us by the epidermis. Strong evidence now shows that microbial communities are in an equilibrium across the basement membrane zone and populate the dermal stroma. These observations show that the surface microbiome is in direct contact with live cells in the dermis, and therefore is in a prime position to influence local and systemic immunity. Importantly, the observation that microbes enter the dermis is not evidence for infection, rather this findings show that products made by microbes are communicating with dermal cells. The overall goal of this proposal is to understand the elements of the skin barrier that regulate microbial entry into the dermis and begin to test the hypothesis that changes in the dermal microbial community result in altered cutaneous immune responses. Our aims are as follows: Aim 1: Understand the role of filaggrin in control of entry of the microbiome and immune modulation. Aim 2: Study how the function of the dermal microbiome is influenced by cathelicidin expression Aim 3: Characterize the dermal microbiome in Atopic Dermatitis before and after skin barrier therapy. Successful completion of these aims will provide new understanding of the skin immune system and can clarify the pathophysiology of inflammatory skin diseases such as atopic dermatitis.